1. Field of the Invention
This invention relates to certain photothermographic elements having increased preexposure storage stability. In one of its aspects it relates to a photothermographic element comprising at least one photosensitive layer comprising a photosensitive component, such as at least one photosensitive silver halide layer, and at least one layer comprising a processing agent for the photosensitive component, with a certain separation polymer between the photosensitive silver halide and the processing agent. In another of its aspects it relates to a process for providing a developed image, such as a silver image and dye image, in a photothermographic element as described. A still further aspect relates to a photothermographic element comprising a diazotype photosensitive material and a separation polymer as described.
2. Description of the State of the Art
It is well known to produce developed images in a photothermographic material. Photothermographic materials are also known as heat developable photographic materials. Photothermographic materials after imagewise exposure are heated to moderately elevated temperatures in the absence of processing solutions or baths to provide a developed image. The heat development can produce a developed image, such as a developed silver image.
In some photothermographic elements it has been useful to have a separation layer between certain layers of the element. The separation layer as used herein is intended to refer to a layer which separates the photosensitive layer from other layers of the element such as a layer containing a processing agent. The separation layer in some instances has provided increased preprocessing stability for the photothermographic element. This is illustrated, for example, in U.S. Pat. No. 3,893,860 of Sutton and Stapelfeldt, issued July 8, 1975 wherein certain acrylamide polymers are described as useful in certain layers of a photothermographic element comprising certain silver salts of heterocyclic thione compounds and photosensitive silver halide. While the acrylamide polymers described provide a degree of improvement in such a photothermographic element, a need has continued to exist to provide further increased stability of a photothermographic element prior to imagewise exposure of the element. The selection of a useful material for such separation layers encounters several problems. In many cases the polymer selected does not satisfy one or more of the characteristics that are desirable, such as (1) providing sufficient resistance to surface bubbling in a photothermographic material upon processing, (2) not significantly adversely affecting sensitometric properties of the photothermographic material, (3) being sufficiently transparent for desired viewing of an image, (4) having sufficient resistance to decomposition at processing temperatures, and (5) providing a sufficient degree of permeability to permit processing materials to diffuse through the element at the desired time and desired rate at processing temperature.
Proposals have been made for separation layers in silver photothermographic elements, as illustrated by the photothermographic element described in U.S. Pat. No. 3,893,860 and the diazotype heat processable materials described in U.S. Pat. No. 3,520,691 of Scheler et al, issued July 14, 1970. Separation layers have also been proposed in processing webs which can be laminated to photothermographic elements. These are described, for example, in U.S. Pat. No. 3,248,219 of Jacobs, issued Apr. 26, 1966 and U.S. Pat. No. 3,576,632 of Bornemisza, issued Apr. 27, 1971. Polymers proposed for separation of the photosensitive component and a processing agent in such materials are not entirely satisfactory for photothermographic silver or diazotype elements because they do not satisfactorily answer one or more of the described problems.
It has been desirable to provide a photothermographic element, especially a photosensitive silver halide photothermographic element and a diazotype photothermographic element, which demonstrates increased stability under storage conditions prior to imagewise exposure and improved control of rate of development upon processing at processing temperatures.